Exhaust silencers for high-powered gas turbines present great technical and financial problems in that, because they must withstand exhaust gases which reach temperatures on the order of 500.degree. C. and which generally contain corrosive compounds, such exhaust silencers must be necessarily made from stainless steel and other valuable and hence expensive materials. Furthermore, the low noise levels required by regulations oblige plant designers to adopt silencers of larger overall dimensions, the cost of which is not only the cost of the equipment, but the total cost with time, i.e., connected to the life of components, and to the costs deriving from the turbine shut down times necessary for maintenance and servicing operations.
Several types of silencers are already known which consist essentially of an outer case of normal steel covered internally with large insulating and soundproofing panels and housing in its interior aligned rows of vertical structures intended for supporting the large soundproofed panels, which dampen the pressure pulsations (and hence the noisiness) of gases passing through them.
Each of said large coating panels, provided both on the inner walls of the outer case and on the inner rows of support structures, is attached to the wall, by either welding or fastening (e.g., by riveting), a large plate of perforated high-temperature-withstanding stainless steel on a large-size rectangular supporting frame containing in its interior a pad of resistant cloth, full of fibrous soundproofing and insulating material such as mineral wood.
Now then, all the silencers of the prior art show severe drawbacks, which substantially lead to a short service life, in turn requiring frequent replacements and consequent frequent shut-down of the turbine unit, adding considerable overall costs.
The flue gases exit the gas turbine exhaust at high temperatures and with considerably high speeds, tubulences and pressure pulsations. This generates high vibrations and resonances on the perforated-plate walls of the panels which, being of large dimensions, are not very rigid. Because of such a poor stiffness, fatigue breakages occur at the weakest points, which are generally the edges of the perforated plates, where they are fastened or welded to the support frames, or in their central points, where the highest deformations occur.
Breakage at the edges of these large panels is also caused by temperature stresses, both under steady-state running conditions, and under transient conditions (turbine start-up and shut-down), resulting from deformations of the support frames for the perforated steel plates. Such deformations are caused by the high differences in temperature which occur between the perforated stainless steel plate, (which has a low thermal inertia and is directly licked at high speed by the hot exhaust gases) and the underlying support frame of iron (which, being very thick in order to accommodate the insulating-soundproofing pad, is directly heated, and shows a high thermal inertia). Furthermore, in case of a wall panel, said support frame is also directly adjacent, on its outer surface, to the cold support structure of the outer case.
From these breakages, which are often not immediately seen from the outside, much more serious consequences follow rapidly, such as the lifting of the edges of the perforated plates, their subsequent detachment and the consequent loss of insulating material.
Another drawback connected to the large dimensions of the generally rectangular panels is that the panels undergo, between low and high temperatures, large dimensional changes, on the order of several centimetres, so that, besides particular and sophisticated fastening systems for said panels having to be adopted to allow such great expansions to occur (often decreasing the rigidity of the whole structure), it also becomes necessary to leave large clearances, of several centimetres, between adjacent panels. These clearances disappear at high temperature, but they are very dangerous under transient running conditions, because the spaces make it possible for the gas to infiltrate between the panels and heat their sides in differential fashion relative to each other. In addition, these large panels require pads filled with insulating and/or soundproofing material, also of large dimensions, so that packing of the fibrous material frequently occurs which, due to the effect of gravity and of the gas pulsations, tends to move material towards the bottom of the pad, thus leaving empty and therefore ineffectual the upper portions, in which the considerable and anomalous temperature increases eventually create conditions which shorten still further the life of the structural parts.
A very serious further drawback of prior art silencers is represented by the difficulty, and near impossibility, of testing the structure of the panels and their contents.
These difficulties are connected with the considerable area of the surfaces to be checked, and the fact that the silencer devices are not manufactured in series according to standardized processes but generally as sets of a few units, often differing from each other because of the different soundproofing requirements and because of the relatively undefined manufacturing techniques in large structural steel manufacturing workshops.
Finally, a further drawback arises from injudicious and expensive use of the insulating and soundproofing material in the thick pads inserted in the wall panels, inasmuch as these panels are filled with one single type of material, which material must perform different functions at different distances from the surface contacting the exhaust gases. In fact, starting from the surface contacting the hot gases, the first portion of material must withstand high temperatures and perform functions both of acoustical damping and of thermal insulation, whereas on the side facing the cold walls of the outer case, the material must only perform insulating functions and the resistance to high temperatures is therefore not required.